you need to check the diameter of the current pipe and the uni backbox. you can then get adaptors to take it up or down what ever is required.

also make sure that the bracket that hooks the back box up is adjustable as it may just look pants when fitted, having it adjustable allows you to let it sit properly.

alternatively get a specific exhaust for your car - they aren't much more and at least you know its designed with your car in mind and is more likely to have a +ve performance outcome rather than -ve on caused by lack of back pressure etc.